It is known in the art relating to robotic and automatic GMAW manufacturing applications that the welding torch contact tip, being a consumable item, is usually replaced between shifts, during maintenance, or when an operator observes unacceptable welding defects on manufacturing workpieces. If the contact tip needs to be replaced during a shift, the entire welding line or cell must be shut down, which results in manufacturing delays and increased costs for the plant.
Judging when to replace a contact tip is a subjective process. Certain operators may pass (accept) some workpieces having welding defects, while other operators may have less tolerance for defects, thereby consuming more contact tips over time. Also, usually when an operator determines that a contact tip needs to be replaced, welding defects have begun to occur, thus meaning that some workpieces may have to be repaired or scraped for quality reasons.
Historically, there have been two major contact tip failure mechanisms that have been observed over the past 50 years in constant voltage (CV) welding applications. The first is excessive wear at the front of the bore in the contact tip, which is known as a “keyhole.” The second failure mechanism is a sudden stoppage of the feeding of electrode welding wire due to a clog and/or excessive feeding force, causing the electrical welding arc to burn back to the contact tip, which is known as “burn back.”
The application of new GMAW welding processes, especially pulse and modified short circuit welding, over the past decade has introduced a new contact tip failure mechanism. The high frequency and high peak current of pulse and modified short circuit welding make that working conditions of the contact tip more critical than in CV welding. In pulse and short circuit welding, the function of the contact tip becomes a “sliding switch,” i.e., while the electrode welding wire slides along the central bore of the contact tip, energy is transferred at dramatically changing (variable) rates. For example, it is common for a 0.9 mm (0.035 inch) solid electrode wire to be welded at 400 Amp peak current in a pulse welding application, while in constant voltage applications the typical welding current is only about 200 to 250 Amps. Also, in pulse welding, the welding current changes from the background current (100 Amps) to the peak current (400 Amps) in 0.15 ms, which corresponds to a rate of 2,000,000 Amps per second. The high welding current and drastic current ramp causes significant arc erosion on the contact tip, thus deteriorating the contact tip. Pulse welding applications are also typically set at high welding speeds, and therefore require accurate delivery of the welding current waveforms to the arc. Thus, consistent performance of the contact tip is more critical than in CV applications. Also, the useful lifespan of a contact tip in pulse applications is significantly shorter than in CV applications. Therefore, it is more likely that the contact tip will have to be replaced during a shift, and knowing when to replace the contact tip is important for preventing welding defects and minimizing operating costs.
Conventional control systems in robotic and automatic welding applications compare the commanded (desired) welding current and voltage with actual measured data. When the difference between the commanded values and the measured data is higher than a certain predetermined threshold, the system will regard the welding as out of compliance and will request immediate action such as shut down of the manufacturing line. However, a monitoring method does not exist that outputs an alarm or other signal prior to the occurrence of welding defects. Thus, there is a need for monitoring contact tip deterioration in real time so that contact tips may be replaced prior to the occurrence of welding defects or at least in a manageable manner such that welding defects and contact tip replacement are kept at a minimum.